In accordance with the recent prevalence of personal computers and the Internet, demands for transmitting information have rapidly increased and thus an optical waveguide is needed in which a whole length thereof is short, a transmitting speed is high and optical transmitting loss is small. The optical waveguide is used as an interconnection for transmitting light and an optical device such as an optical splitter (an splitting/coupling device) is indispensable as an essential element of the optical waveguide. It should be noted that the optical splitter (splitting/coupling device) activates as an optical coupler when a direction of transmitting light is inverted.
Conventionally, a one-to-N (1×N; N is 2-8.) split type optical splitter made of optical waveguides has been provided by arranging one-to-two (1×2) split type optical splitters in a tree form. In the following explanation, for convenience, a one-channel side of the splitter is referred to as an input port while a N-channel side thereof is referred to as output ports. However, the splitter can be used regarding either an optical splitting function or an optical coupling function.
Conventionally, in a one-to-four (1×4) or one-to-more split type optical splitter, an S-bent optical waveguide is disposed between a splitting element at a first stage from the input port and a splitting element at a second stage therefrom to connect them to each other, a direction of an input part of the S-bent waveguide and a direction of an output part thereof being parallel to each other. Similarly, in a one-to-eight (1×8) or one-to-more split type optical splitter, such an S-bent optical waveguide is disposed between a splitting element at a second stage from the input port and a splitting element at a third stage therefrom to connect them to each other.
A multiple split type optical circuit has been proposed in the prior art; which has at least two Y-type split elements disposed between an input path at a first stage and each of output paths at a final stage, and is arranged in a tree form parallel to directions of normal lines of the input path and the output paths; in which light transmitted into the input path is equally split to the output paths; and in which each of the Y-type split elements from a second stage to a final stage on the outermost side relative to a reference line, which is an extended centerline of the input path at the first stage, is outward, and obliquely arranged so that an angle θ between a centerline of each Y-type split element and the reference line increases when the split elements are viewed from the second stage to the final stage (Please see Patent Publication 1 below).
Another optical waveguide structure in the prior art has a waveguide which is split from an input part thereof in a tree form and includes a splitting curved line having a curvature radius enough to prevent transmission of light (Please see Patent Publication 2 below).
In the prior art, an optical coupler has an input port, 2n output ports and a plurality of waveguides arranged in a substantially coplanar relationship; wherein the waveguides include Y-type junctions and a plurality of waveguide sections, each of the waveguide sections optically connecting an input part of a Y-type junction to one of two output parts of another Y-type junction; wherein the Y-type junctions are arranged in a tree and branch form along the waveguides; wherein a first Y-type junction (J21) having an output path non-parallel to the output port of the optical coupler is connected to a second Y-type junction (J31, J32) via a curved waveguide section without singular points; and wherein an inflection point on any one of the waveguides disposed between the input port and Y-type junctions at a nth stage is located at one of the n Y-type junctions along a passage of the waveguide (Please see Patent Publication 3 below).
Another optical coupler in the prior art has one input optical waveguide and N output optical waveguides; wherein one optical incident signal from the input waveguide is split into N optical signals which are then output to the respective output waveguides; wherein m stages of Y-type junction waveguide elements are arranged by using a plurality of Y-type waveguide elements, one optical incident signal being split into two optical signals in each of the Y-type waveguide elements; wherein a plurality of curved waveguides are alternatively connected to the Y-type waveguide elements and at least one curved waveguide is connected between the Y-type waveguide element at the mth stage and the output waveguide; wherein, when a direction in which an optical signal is transmitted is considered to be a longitudinal ordinate and a contact point of the input waveguide contacting with the Y-type waveguide element at the first stage is considered to be an original point, the Y-type waveguide elements and the curved waveguide are arranged and dimensioned so that a path having the maximum length among the N paths from the original point to the output waveguides is replaced with a path having a minimum length (See Patent Publication 4).
Patent Publication 1: Japanese Patent No. 3030108
Patent Publication 2: Japanese Patent Laid-open Publication No. 51-124940
Patent Publication 3: Japanese Patent Laid-open Publication No. 5-196826
Patent Publication 4: Japanese Patent Laid-open Publication No. 2002-530690